Thin film transistor liquid crystal displays (TFT-LCDs) have undergone a long period of basic research, and after realising large-scale production and commercialisation, thin film transistor liquid crystal displays have become mainstream products in LCD applications due to the advantages of light weight, being environmentally friendly, high performance, etc., thereof: the application of TFT-LCD can be seen everywhere whether in small-sized mobile phone screens, large-sized notebook PCs or monitors or in large-sized liquid crystal televisions (LCD-TV).
Early commercial TFT-LCD products basically relate to using a TN display mode, and the largest problem thereof is a narrow viewing angle. With the increase in product size, especially the application in the TV field, an IPS display mode and a VA display mode, which have the characteristic of a wide viewing angle, have been sequentially developed and applied; in particular, based on the improvement of the VA display mode, the breakthrough development thereof has been achieved successively in major companies, which mainly depends on the advantages of a wide viewing angle, a high contrast, no need for frictional alignment, etc., of the VA mode itself. Furthermore, the contrast of the VA mode display is less dependent on the optical anisotropy (Δn) of the liquid crystal, the thickness of the liquid crystal cell (d) and the wavelength (λ) of the incident light, which will necessarily make the VA mode become a very promising display technique.
However, the liquid crystal medium used in an active matrix addressing mode display element for the VA mode, etc., itself is not perfect. The defects, for example, the residual image level is significantly worse than that of a positive dielectric anisotropic display element, the response time is relatively slow, and the driving voltage is higher. At this point, some new types of VA display techniques have quietly emerged: for example, a PSVA technique realises a wide viewing angle display mode similar to that of MVA/PVA, and also simplifies a CF process, such that the aperture ratio is increased while lowering the CF cost; furthermore, a higher brightness is obtained, thereby obtaining a higher contrast. In addition, since the liquid crystal of the entire panel has a pretilt angle, there is no domino delay phenomenon, a faster response time can also be obtained while maintaining the same drive voltage, and the residual image level will also not be affected; however, due to Fine Slit densely distributed electrodes in pixels, if the electrode width cannot be evenly distributed, the problem of uneven display can easily occur. Like a UVVA technique, on the basis of keeping the advantages of the PSVA technique, since there is no Slit structure on the TFT side, the problem of display unevenness caused by uneven pixel electrode width is also improved. Although the display device is continuously developing, people are still always devoted to studying new liquid crystal media, such that liquid crystal media and the performances of display devices in which the liquid crystal media are used can continuously advance forward.
Polymerizable mesogenic units (RMs) are currently a very popular and important topic in the display industry, and possible application fields thereof include polymer stabilized alignment (PSA) liquid crystal display, polymer stabilized blue-phase (PS-BP) liquid crystal display, pattern retarder films, etc.
The PSA principle is being applied to different typical LC displays such as PSA-VA, PSA-OCB, PS-IPS/FFS and PS-TN liquid crystal displays. Taking the most widely used PSA-VA display as an example, the pretilt angle of the liquid crystal cell can be obtained by a PSA method, and the pretilt angle has a positive effect on the response time. For PSA-VA displays, standard MVA or PVA pixel and electrode designs can be used; however, if a specially patterned design is used the electrode design on one side and no protrusion design is used on the other end, the production can be significantly simplified while the display is imparted with a very good contrast and a very high light transmittance.
It has been found in the prior art that LC mixtures and RMs still have some disadvantages in applications in PSA displays. First, so far not every desired soluble RM (polymerizable mesogen or polymerizable compound) is suitable for use in PSA displays; in addition, if it is desired to carry out a polymerization by means of a UV light without the addition of a photoinitiator (which may be advantageous for some applications), the choice becomes narrower; furthermore, a “material system” formed from an LC mixture (hereinafter also referred to as an “LC host mixture”) in combination with the selected polymerizable component should have the lowest rotational viscosity and the best photovoltaic performance for increasing the “voltage holding ratio” (VHR) to achieve effects. In terms of PSA-VA, a high VHR after irradiation using a (UV) light is very important; otherwise, the problems of the occurrence of residual images to the display, etc., may be finally caused. So far, not all combinations of LC mixtures and polymerizable components are suitable for PSA displays. This is mainly due to the effects in the aspects of the UV-sensitive wavelength of polymerizable units being too short, or no tilt angle or an insufficient tilt angle occurring after light irradiation, or the polymerizable component having a poorer homogeneity after light irradiation, or the VHR after UV is lower for TFT display applications.
PS(A)-displays disclosed in the prior art generally comprise RM, wherein a ring system of the mesogenic group is bonded in para-position to the adjacent group (other ring, a bridging group, a spacer or a polymerizable group) thereto; for example, a display proposed in the publication of the invention patent with publication number EP 1498468 A1 comprises RM selected from the following formulas:
wherein P1 and P2 represent a polymerizable group such as an acrylate group, a methacrylate group, vinyl, vinyloxy or an epoxy group. However, the RMs as described in the above formulas generally have the problems of a high melting point and a limited solubility in many currently used liquid crystal mixtures, thus leading to ease of often spontaneously crystallising from the mixture.
In the publication of the invention patent with the publication number CN 101848978 A, a structure of formula Ra-(A1-Z1)m1-(A2-Z2)m2-(A3)m3-Rb I is proposed. The main improvement point thereof is that A1 and A3 each independently represent 1,3-phenylene, naphthalene-1,3-diyl, naphthalene-1,6-diyl, naphthalene-2,5-diyl or naphthalene-2,7-diyl, and compared to the RMs in the prior art, it shows a lower melting point, a lower crystallisation tendency and an improved solubility in many commercially available liquid crystal host mixtures.
However, it was found in the study that the RM monomer further has an important property, that is, the ratio of polymerization conversion, i.e., the rate of polymerization, within the same time under the irradiation of the same light intensity of UV light according to the panel process without adding any photoinitiator has an important effect on the panel yield. If the rate of polymerization is slow, the RM conversion will be incomplete and the effect of PSA (polymer stabilized alignment) will not be achieved; moreover, the liquid crystal medium is susceptible to deterioration when exposed to the UV light for a long time. If the rate of polymerization is too fast, polymerization particles will be too large, causing zara particles, which seriously affects the panel yield.
Thus, there has always been a great demand for PS(A)-displays, especially of VA and OCB types, and for liquid crystal media and RMs for use in these displays. In addition, these displays do not show the above-mentioned defects or only show the above-mentioned defects to a small extent and have improved properties. In particular, there is a great need for PS(A)-displays and liquid crystal media and RMs for use in such displays. As well-known and acknowledged, they need to have a high stability to prevent demixing at low temperatures, a high resistivity, a wide operating temperature range, a short response time (even at low temperatures), a low threshold voltage (which makes a large amount of grey scale, a high contrast, and a wide viewing angle possible), and a high “voltage holding ratio” (HR) value after UV exposure. In addition to the above-mentioned well known and acknowledged requirements, an RM monomer and a liquid crystal medium containing the RM monomer are also required to have an appropriate rate of polymerization to avoid the occurrence of panel failure.
The rate of polymerization of the RM monomer is closely related to the UV absorption spectrum thereof, and the main influence factors therefor are the main structure thereof, substituent groups, etc., wherein changes and effects brought about by different substituent groups are unpredictable. Slight changes in the RM structure may have a significant effect on the performance thereof.